1. Technical Field
The present invention relates to a nonvolatile memory device, and more particularly, to a program method of a nonvolatile memory device.
2. Related Art
Semiconductor memory devices are generally divided into a volatile memory device and a nonvolatile memory device. While the volatile memory device loses stored data when power supply is interrupted, the nonvolatile memory device can retain stored data even though power supply is interrupted. Nonvolatile memory devices include various types of memory cell transistors where the category of devices may be divided based upon a structure of a memory cell transistor. Nonvolatile memory devices are divided into a flash memory device, a ferroelectric RAM (FRAM), a magnetic RAM (MRAM), a phase change RAM (RAM).
Among nonvolatile memory devices, flash memory devices are generally divided into a NOR flash memory device and a NAND flash memory device, depending upon a connection state of memory cells and a bit line. The NOR flash memory device has a structure in which two or more memory cell transistors are connected in parallel to one bit line. Accordingly, the NOR flash memory device has excellent characteristics in terms of random access time. Conversely, the NAND flash memory device has a structure in which two or more memory cell transistors are connected in series to one bit line. This structure is referred to as a cell string structure. One bit line contact is needed per a cell string. Therefore, the NAND flash memory device has excellent characteristics in terms of the degree of integration.
The memory cells of a flash memory device are divided into on cells and off cells depending upon a threshold voltage distribution. The on cells are erased cells, and the off cells are programmed cells. In order to program a memory cell of a flash memory device, a ground voltage (that is, 0V) is applied to the bit line which is connected to the memory cell, and a high voltage is applied to the word line which is connected to the memory cell. As these voltages are applied to the bit line and the word line of the memory cell to be programmed, the memory cell is programmed through F-N (Fowler-Nordheim) tunneling. Such a program method of a flash memory device is disclosed in detail in Korean Patent No. 10-0842758 which is incorporated herein by reference.